The present invention relates to flat woven papermaker""s forming fabrics having a paper side layer and a machine side layer interconnected by machine side layer weft binder yarn pairs. Within the overall fabric weave pattern, the number of machine side layer weft yarns between each pair of weft binder yarns can zero, one, two or three.
Flat woven papermaker""s forming fabrics in which paper side layer weft binder yarn pairs are used to interconnect the weave structures of the paper and machine side layers are well known. Various arrangements have been described, for example by Wilson, U.S. Pat. No. 5,518,042; Vohringer, U.S. Pat. No. 5,152,326; Quigley et al., U.S. Pat. No. 5,520,225; Ostermayer et al., U.S. Pat. No. 5,542,455; Wright, U.S. Pat. No. 5,564,475; Wilson, U.S. Pat. No. 5,641,001; Ward, U.S. Pat. No. 5,709,250; Seabrook et al., U.S. Pat. No. 5,826,627; and Wilson, U.S. Pat. No. 5,937,914. Many others are known. None of these references discuss in any detail the impact of the use of weft binder pairs on the properties of the machine side layer.
As used herein, the following terms have the following meanings.
The term xe2x80x9cweft binder yarnxe2x80x9d refers to each yarn of a pair of yarns which together occupy a single unbroken weft path in the machine side layer, and which separately interweave with a paper side layer warp yarn.
The term xe2x80x9cinterweavexe2x80x9d refers to a locus at which a yarn forms at least one knuckle with another yarn in the paper side layer.
The term xe2x80x9cinterlacexe2x80x9d refers to a locus at which a yarn forms at least one knuckle with another yarn in the machine side layer.
The term xe2x80x9csegmentxe2x80x9d refers to a locus at which a weft binder yarn interlaces with at least one machine side layer warp within the machine side layer.
The term xe2x80x9cfloatxe2x80x9d refers to that portion of a yarn which passes over, or under, a group of other yarns in the same layer of the fabric without interweaving or interlacing with them. The associated term xe2x80x9cfloat lengthxe2x80x9d refers to the length of a float, expressed as a number indicating the number of yarns passed over, or under, as appropriate. A float can be exposed on the machine side or paper side of each of the paper side layer and the machine side layer. The term xe2x80x9cinternal floatxe2x80x9d thus refers to a float exposed between the two layers, either on the machine side of the paper side layer, or on the paper side of the machine side layer.
The terms xe2x80x9csymmetryxe2x80x9d and xe2x80x9casymmetryxe2x80x9d, and the associated terms xe2x80x9csymmetricalxe2x80x9d and xe2x80x9casymmetricalxe2x80x9d, refer to the shape of the path occupied by a weft binder yarn as it exits the machine side layer, interweaves with a paper side layer warp, and enters the machine side layer. The path is symmetrical when the interweaving point is located substantially at the middle of the path, and the number of warp yarns between the exit point and the interweaving point is equal to, or nearly equal to, the number of yarns between the interweaving point and the entry point.
The notation such as 3/2 in reference to a fabric design refers to the number of warp, or machine direction yarns, over or under which a weft, or cross machine direction yarn, floats within the weave pattern. Thus 3/2 means that a weft yarn floats over three warp yarns and then under two warp yarns within the weave pattern.
The prior art, as exemplified above, seems to have limited the designs of forming fabrics of this type to those in which weft binder yarn pairs are used to provide an intrinsic component of the paper side layer weave design, and to enhance the paper side layer formation characteristics, as in the Wilson and Seabrook patents. The prior art designs also created limitations which were generally believed to be necessary to maximise fabric stability, reduce or even eliminate sleaziness (the movement of one of the two layers relative to the other) and fabric delamination (the catastrophic separation of the two layers caused by both internal and external abrasion of the weft binder yarns). The prior art generally served to restrict the number of paper side layer and machine side layer weave designs that could be combined together. It is thus apparent that a great deal of experimental effort had to be expended in order to find compatible combinations of paper and machine side layer weave designs capable of interconnection by means of weft binder yarns, due to the restrictive criteria noted above.
This invention is based on the discovery that machine side layer weft yarns can be successfully used as weft binder yarn pairs in fabrics of this type. The machine side layer weft binder yarn paths can also be chosen to minimise internal stresses introduced during weaving the two layer fabric. Further, their use also appears to provide significantly greater flexibility in the choice of compatible paper side layer and machine side layer weave designs. In this invention, within the weave pattern repeat, there is either zero, one, two or three machine side layer weft yarns between each pair of machine side layer weft binder yarns. It is thus possible to match the locations of the internal floats of the weft binder yarns within the machine side layer pattern repeat to the desired paper side layer interweaving locations, so that they are located more or less at the midpoints of the paper side layer internal warp floats. The paper side layer weave design is selected so as to be appropriate for the paper product to be made using the forming fabric. It is also now possible to select the machine side layer weave design to optimise machine side layer properties, and then to select interweaving points that are located more or less at the midpoints of the internal floats of the weft binder yarns. It has also been discovered that not all of the available interweaving locations have to be used: it is possible to leave some of them out within the forming fabric weave pattern repeat.
In the fabrics of this invention, the paper side layer internal warp float should be as long as possible, with the interweaving point located as close as possible to the middle of this float. The path occupied by the machine side layer weft binder yarn internal float should be as symmetrical as possible about the interweaving point. Further, in the fabrics according to this invention all of the machine side layer weft yarns are substantially the same size, and therefore although at least some, if not all, are doubled as weft binder yarn pairs, all of them contribute to the properties of the machine side layer of the fabric. The paper side layer weft yarns will frequently be smaller than the machine side layer weft yarns, and may also be larger.
The interweaving locations of the paper side layer and machine side layer floats should be chosen with some care. The limitation on both of these floats appears to be that each should be as long as is reasonably possible. In its path in between the two layers, the machine side layer weft float has essentially a xe2x80x9cVxe2x80x9d shape: as the float length increases, the V is flattened reducing the out of plane stresses. If the V shaped path is not symmetrical, or the float is relatively short, any stresses imposed on the forming fabric are increased at the shorter end of the float. The upper limits on these two float lengths cannot be directly determined.
The present invention seeks to provide a papermaker""s forming fabric comprising in combination a paper side layer including a first set of warp and weft yarns interwoven according to a first pattern which provides for internal floats of the paper side layer warp yarns, a machine side layer including a second set of warp and weft yarns, in which the weft yarns include weft binder yarn pairs, interwoven according to a second pattern which provides for internal floats of the machine side layer weft binder yarns, wherein within the fabric weave pattern repeat:
(i) the weft binder yarn pairs together occupy successive segments of an unbroken weft path within the machine side layer;
(ii) at least some of the machine side layer weft binder yarn internal floats interweave with paper side layer internal warp yarn floats;
(iii) there is zero, one, two or three machine side layer weft yarns between each pair of binder yarns; and
(iv) the paper side layer warp yarn internal float length is at least 2.
Preferably, within the weave pattern repeat, the number of machine side layer weft yarns between each pair of weft binder yarns is constant. Alternatively, within the weave pattern repeat, the number of machine side layer weft yarns between each pair of weft binder yarns is not constant.
Preferably, the segments of the weft binder yarn unbroken weft path occupied by each member in succession are the same length. Alternatively, the segments of the weft binder yarn unbroken weft path occupied by each member in succession are not the same length.
Preferably, each member of a weft binder yarn pair interweaves at or near to the midpoint of an internal paper side layer warp yarn float.
Preferably, within the pattern repeat, the majority of the paper side layer warp yarns interweave once with a machine side layer weft binder yarn.
Preferably, the path occupied by each weft binder yarn, as it passes from interlacing with the machine side layer warp yarns in a segment of the machine side layer weft yarn path to interweave with a paper side layer warp yarn internal float and returns to interlace with the machine side layer warp yarns in another segment of the machine side layer weft yarn path, is more or less symmetrical about the interweaving point.
Preferably, the paper side layer warp yarn internal float length is at least three. Most preferably, the paper side layer warp yarn internal float length is four or more.
Preferably, the paper side layer is woven according to a weave design chosen from the group consisting of: a 2/1 twill, a 2/1 broken twill, a 2/1 satin, a 2/2 basket weave, a 2/2 twill, a 3/1 twill, a 3/1 broken twill, a 3/1 satin, a 3/2 twill, a 3/2 satin, a 4/1 twill, a 4/1 broken twill, a 4/1 satin, a 5/1 twill, a 5/1 broken twill, and a 5/1 satin.
Preferably, the machine side layer is woven to a weave design chosen from the group consisting of: a plain weave, a 2/1 twill, a 2/1 broken twill, a 2/1 satin, a 2/2 basket weave, a 3/1 twill, a 3/1 broken twill, a 3/1 satin, a 3/2 twill, a 3/2 satin, a 4/1 twill, a 4/1 broken twill, a 4/1 satin, a 5/1 twill, a 5/1 broken twill, a 5/1 satin, a 6/1/ twill, a 6/1 broken twill, a 6/1 satin, and an Nxc3x972N design as disclosed by Barrett in U.S. Pat. No. 5,544,678.
Preferably, the ratio of the number of paper side layer weft yarns to the number of machine side layer weft yarns is chosen from the group consisting of: 1:1, 3:2, 5:3, 2:1 or 3:1, when the weft binder yarns are included, and a pair of weft binder yarns counted as one machine side layer weft yarn.
Preferably, the ratio of the number of paper side layer warp yarns to the number of machine side layer warp yarns is 1:1. Alternatively, the ratio of the number of paper side layer warps to the number of machine side layer warps is 2:1.
Both the paper side layer and the machine side layer may be woven according any known weave design which would be acceptable for the intended use of the fabric, with the proviso that the paper side layer must be woven according to a design which provides for an internal warp float length of at least 2, and desirably it is at least 3 or more, since it is then possible to find more acceptable interweaving locations for the weft binder yarns.
Preferably, the fabrics of this invention have a 5/1 broken twill paper side layer weave which provides for a paper side layer warp internal float length of five yarns, and a 2/1 twill machine side layer design.